contrib/dbmirror 7.4 README update

Here is an update to the dbmirror README file.

-References to older versions of PostgreSQL have been removed(It no longer
compiles against older versions)
-Added a link to PgPerl at GBorg.

Thanks

--
Steven Singer                                       ssinger@navtechinc.com
Dispatch Systems                            Phone:  519-747-1170 ext 282
Navtech Systems Support Inc.                AFTN:   CYYZXNSX SITA: YYZNSCR
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